In a radiofrequency signal emitting/receiving system, the reception sequence comprises, in a known manner, an antenna receiving the signal that is transmitted to the amplification circuit of a receiver whose function is to filter and amplify the signal that is representative of the transmitted information. The amplifier's main role is to adjust the signal to an appropriate level for the demodulation circuit by reducing the degradation of the signal/noise ratio and the distortion provided by amplifying the useful signal. In receivers for wireless telecommunication infrastructures, there is a major need for highly linear, very-low-noise amplifiers. Furthermore, these amplifiers must be capable of adapting to the requirements of different receiver manufacturers, as these receivers do not always require the same performance compromises. Additionally, various terrain configurations make easily adjustable performances a necessity in order to address the majority of cases. For example, signal losses within the cable that connects the antenna placed at the top of the mast or pylon to the receiver placed in a housing within the base station depend on the length of that cable. For another example, the filter's characteristics may differ from one manufacturer to another.
The receiver must have a low noise figure (or “NF”) and an adjustable power gain in order to adjust the signal's level at the input of the demodulator and to guarantee the broadest possible range of gain variation as appropriate for the terrain configuration. To that end, a low-NF, variable-gain amplifier is inserted into the reception sequence right after the antenna, cable, and filter. The noise from the amplifier must be low enough to add little noise to the reception chain, and the amplifier must exhibit a linearity and output power which are sufficient to enable the simultaneous amplification of high-level and low-level amplitude signals without distortion, throughout the entire range of the variation being considered. Furthermore, among some manufacturers, the option to adjust the amplifier's gain makes it possible to offset temperature variations within the reception sequence in order to maintain a roughly constant gain value.
One currently known solution that is particularly used in television tuners is made up of a linearly variable attenuator, followed by a variable-gain amplification stage placed after a low-noise, fixed-gain amplifier and a mixer. This solution, using MOSFET (Metal-Oxide Semiconductor Field Effect Transistor) technology, makes it possible to have a constant output gain level while limiting the noise figure's degradation. This is appropriate to the application for receiving television signals. However, the performances achievable through this sort of solution are not appropriate to the needs in the field of mobile telecommunication infrastructures.
Today, the performances of a low-noise, variable-gain amplifier for a base station, in the range around 900 MHz for example, normally have a noise figure of less than 1 dB, a gain of about 30 dB, and an intermodulation product of about 3 at its input above 0 dBm across the entire temperature range, and for a gain variation range of about 15-20 dB.